1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image by projecting light onto a rotating polygonal mirror.
2. Description of the Related Art
Image forming apparatuses, such as laser printers, copiers, facsimile apparatuses and the like, having a plurality of electrophotographic image forming units, have been known as conventional image forming apparatuses (for example, refer to Japanese Patent Application Laid-Open (Kokai) No. 07-123195 (1995)). Each image forming unit of such an image forming apparatus includes a semiconductor-laser unit for projecting a light beam, for example, modulated with image data onto a photosensitive member, a rotating polygonal mirror for performing main scanning on the photosensitive member by deflecting the laser beam from the semiconductor-laser unit by being rotatably driven by a polygonal-mirror driving motor, a PLL (phase-locked loop) control unit for controlling the revolution speed of the polygonal-mirror driving motor based on a reference-frequency signal, and a synchronism sensor for generating a main-scanning synchronizing signal (BD signal) by receiving reflected light from each mirror surface of the rotating polygonal mirror. By controlling exposure scanning on the photosensitive member with a laser beam corresponding to image data based on an output timing of a predetermined image-writing enabling signal and an output timing of the main-scanning synchronizing signal, images formed on respective image forming units are superposed without producing position deviation (for example, refer to Japanese Patent Application Laid-Open (Kokai) No. 09-292582 (1997)).
Since an image forming apparatus having a plurality of image forming units forms a color image by superposing toners of a plurality of colors, a position control technique that is more precise than for a monochromatic (black-and-white) printer is required.
In a color-image forming apparatus, a sheet conveying speed, i.e., an image forming speed (process speed) is sometimes changed depending on the type of a sheet, environment and the like. However, since a polygonal-mirror driving motor usually performs the above-described control, there is the possibility that rotation non-uniformity increases if the revolution speed of the polygonal-mirror driving motor is changed, and particularly in a color-image forming apparatus, the quality of a formed image is degraded.
Accordingly, when changing the sheet feeding speed, image formation is usually performed by skipping some lines without changing the revolution speed of the polygonal-mirror driving motor, i.e., the rotation speed of the rotating polygonal mirror. For example, when the sheet conveying speed is changed to 1/2 of the original speed, scanning is performed by skipping one line in two lines. When the sheet conveying speed is changed to 1/4 of the original speed, scanning is performed by skipping one line in four lines (for example, refer to Japanese Patent Application Laid-Open (Kokai) No. 07-322022 (1995)).
When forming a monochromatic (black-and-white) image by skipping some lines while reducing the sheet feeding speed, a timing of skipping some lines will not cause a problem.
However, in a color-image forming apparatus in which images of predetermined colors are formed by a plurality of image forming units, and the images of the respective colors are transferred onto a recording sheet in a superposed state, if image formation is performed without taking into consideration of a timing of line skipping in image forming units of respective colors at stages posterior to an image forming unit of a reference color, there is the possibility that images of respective colors deviate within ±1 line (when skipping one line in two lines).